This invention is in the field of carbon nanomaterials, in particular, methods for making carbon nanotubes and carbon nanotube fibers.
The term “carbon nanomaterials” is used generally herein to refer to any substantially carbon material containing six-membered rings that exhibits curving of the graphite planes, sometimes by including five-membered and/or seven membered rings amongst the hexagons formed by the positions of the carbon atoms, and has at least one dimension on the order of nanometers. Examples of carbon nanomaterials include, but are not limited to, fullerenes, single-walled carbon nanotubes (SWNTs), double-walled carbon nanotubes, multiple-walled carbon nanotubes (MWNTs), nanotubules, and nested carbon structures with dimensions on the order of nanometers.
Different carbon nanomaterials have different potential applications. Nanotube materials can exhibit extraordinary mechanical, thermal, electrical and/or chemical properties, which have stimulated substantial interest in developing applied technologies exploiting these properties. These technologies include, but are not limited to, advanced small-scale circuitry, body and vehicle armor, drug delivery, high-efficiency conductors, nanocomposite materials, medical applications, automobile and aircraft frames, heat transfer and large scale structures (such as very long single-span suspension bridges and the space elevator).
For example, nanotubes can have very high Young's modulus values. Multi-walled carbon nanotubes have been measured to have Young's modulus values between 0.1 and 1.33 TPa, with the Young's modulus being dependent upon the degree of order within the tube walls (Demczyk et al., 2002, Mater. Sci. and Engr. 1334, 173-178; Salvetat et al., 1999, Appl. Phys. A 69, 255-260). While the art recognizes significant potential for commercial application of carbon nanomaterials, the high cost and difficulty in obtaining these materials in the large amounts necessary for developing these applications has been a major impediment in practical application of these materials, although significant breakthroughs in manufacturing technique allow for kg quantities of short (<100 microns) multi-walled tubes to be made available in today's market.
Carbon nanotubes (CNTs) have been grown by chemical vapor deposition (CVD) methods. The carbon nanotube can be grown on a cluster containing metal atoms which serves as both a base and a catalyst for the high temperature decomposition of a carbon-containing material such as acetylene, ethylene, methane, carbon monoxide, etc. The clusters may be supported on a substrate such as alumina or silica or introduced into the feedstock to make vapor-grown carbon fibers. The clusters may be metal clusters or clusters which combine a metal with another element (e.g. oxides).
As described by Moisala et al. (Moisaal, A. et al., 2003, J. Phys. Condens. Matter 15 S3011-S3035), basic steps in the growth process for single walled nanotubes include catalytic decomposition of the carbon precursor molecules on the surface of the metal-containing catalyst particles followed by diffusion of the carbon atoms into the particles. Supersaturation of carbon in the metal results in solid carbon precipitation from the particles. The physical form of the precipitated carbon depends on several parameters, including catalyst particle size and precipitation rate. The growth mechanism is generally classified as either root growth (also called base growth) and tip growth. In base growth, the catalyst particle stays pinned at the support surface. The tip growth mechanism takes place when the catalyst is lifted off from the support surface due to weak support-catalyst interaction.
The size of the metal-containing cluster can influence the nanotube structure. U.S. Pat. No. 6,692,717 to Smalley et al. reports that metal clusters in the range of 0.5 to 3 nm will produce single-wall nanotubes while larger clusters tend to produce multi-wall nanotubes with outer diameters greater than about 3 nm. Molecular dynamics calculations show that larger metal clusters, which contain at least 20 iron atoms, nucleate SWNTS that have a far better tubular structure than SWNTs nucleated from smaller clusters (Feng D., A. Rosen, and K. Bolton, 2004, J. of Chem. Phys., 121, 2775-2779).
Aligned arrays of single-walled nanotubes have been reported in the scientific literature. Hata et al. report water-assisted catalytic growth of aligned single-walled nanotube “forests” (Hata, K., et al, 2004, Science, 306, 1362-1363). Millimeter-scale nanotube lengths were reported for a growth time of ten minutes. In such a dense nanotube forest, the rate at which the free carbons reach the catalyst may decrease due to mass transport limitations.
Aligned arrays of multi-walled carbon nanotubes have also been reported. Aligned arrays of multi-walled carbon nanotubes have been grown on nickel-coated glass (Zen, R. F. et al., 1998, Science, 282(5391), 1105-1107). For applications requiring good alignment, nanotube diameters greater than 50 nm were recommended. Lengths from 0.1 to 50 microns were reported. Aligned multi-walled carbon nanotubes have also been grown using iron nanoparticles embedded in mesoporous silica (Li, W. S. et al., 1996, Science, 274(5293), 1701-1703). The carbon nanotubes were reported to have diameters of about 30 nm, with spacing between the tubes of about 100 nm and lengths up to 50 microns. Furthermore, aligned multi-walled carbon nanotubes have been grown using oxidized patterned iron films on oxidized nanoporous silicon (Fan, S. et al., 1999, Science, 283(5401), 512-514). Typical nanotube diameters were reported as about 16 nm, with lengths between about 35 and 240 microns.
Furthermore, Japanese Patent Application Publication Number 2004-292310 describes growth of nanotubes by passage of synthesis gas through a nano-porous substrate, where the pore size of the substrate is not larger than the diameter of the nanotube. A catalyst metal is included within the pores or may be vapor-deposited on the substrate.
When carbon nanotubes are grown in an environment of carbon-containing materials, decomposition of the carbon-containing material can cause other carbon structures, such as turbostratic graphite and amorphous carbons, to grow. These other structures can cause inactivation of the catalyst (U.S. Pat. No. 6,692,717). A variety of techniques have been used to reduce the amount of the other structures formed. Hata et al. (ibid) report that including water vapor in the growth atmosphere leads to nanotubes free from amorphous carbon and metal particles. U.S. Patent Application Publication No. 2002/0172767, to Grigorian et al. reports that growth in the temperature range from about 760° C. to about 800° C. limits the formation of amorphous carbon.
A number of laboratories have reported “spinning” mixed-length carbon nanotubes into fibers in much the same way as cotton or wool fibers are spun into thread and yarn. Coagulation-based carbon nanotube spinning has been used to make fibers from single-walled nanotubes (Dalton, A. B. et al., 2003, Nature, 423, 703). The tensile strength of these composite fibers was reported as 1.8 GPa. Spinning by winding a nanotube aerogel onto a rotating rod was reported by Li et al. (Li, Y.-L., et al., 2004, Science, 304(5668), 276-278). The fibers were reported to have a range of strengths between 0.05 N/Tex and 0.5 N/Tex (estimated to be equivalent to about 0.10 and 1.0 GPa). Spinning has also been achieved by drawing carbon nanotubes from a multi-walled nanotube “forest” (Jian, K. et al., 2002, Nature, 419, 801; Zhang, M., et al., 2004, Science, 306, 1358-1361). Zhang et al. (ibid.) report formation of single ply, two-ply, four-ply, knitted, and knotted multi-walled nanotube yarns. Tensile strengths were reported as 150 and 300 MPa for the single ply yarns and between about 250 and 460 MPa for the two-ply yarns.
There remains a need for improved methods for producing long aligned nanotubes by catalytic decomposition of carbon-containing gases. These nanotubes can be used to make nanotube fibers, twine or braid. Because of their length, these nanotubes may also be used in applications that do not require that they be combined into such grouped strands.